On a side surface of the conventional developer cartridge are provided a contact protrusion extending radially outward from a prescribed shaft part, a sensor gear that rotates together with the contact protrusion about the shaft part, and a plurality of gears engaged with the sensor gear that transmit a drive force to a developing roller, an agitator, and the like. When a new developer cartridge is mounted in the body of the image-forming device, the contact protrusion press against one end of a sensor actuator disposed in the device body. The pressure by the contact protrusion pivots the sensor actuator, and a sensor detects this movement, enabling the image-forming device to determine that a new developer cartridge has been mounted.